This invention is directed to polyimide blend polymers and films and to a process for their preparation.
Aromatic polyimide films are widely used in microelectronics fabrication, in devices and packaging, and as dielectric barriers owing to their excellent thermal, mechanical and electrical properties. Many polyimide films have been prepared by structural variation of the aromatic tetracarboxylic acid anhydride component and even more structurally variable aromatic diamine component. Some combinations of aromatic tetracarboxylic acid anhydrides and aromatic diamines exhibit properties useful in a wide range of applications. However, only a few polyimide films have been produced in commercial quantities. It is generally the rule that if a specific polyimide film exhibits a significant improvement in one property, it does so at the expense of some other property.
The properties of polyimide films can be altered by copolymerizing different sets of tetracarboxylic acid anhydrides and diamines or multiples of them. However, this route can be impractical due to higher cost, chemical complexity and slower film production throughput.
For example, U.S. Pat. No. 5,166,308, issued on Nov. 24, 1992, discloses an aromatic copolyimide film for use as an advanced electronic substrate prepared by chemical conversion of a copolyamic acid solution obtained by copolymerization of pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-diaminodiphenylether (DADE) and p-phenylenediamine (PPD). The aromatic copolyimide film advantageously has a low coefficient of thermal and hygroscopic expansion, low water absorption and high mechanical strength. However, during manufacture the copolyimide film exhibits undesirable blister formation during drying and curing, which severely limits production throughput.
An alternative approach, which involves blending of independently prepared polyimides, can sometimes be used to tailor properties of the polyimide film for a specific application. Unfortunately, polyimides, like most polymers, are generally immiscible with each other. When dissolved in a suitable mutual solvent, the blended polyimides tend to phase separate when dried rapidly thereby forming a hazy, coarse surfaced polyimide film, which is undesirable for electronics applications.
For example, Japanese patent publication 01-110535, published Apr. 27, 1989, and Japanese patent publication 6-55843, published Jul. 27, 1994, disclose surface roughened polyimide films prepared by blending two different aromatic polyamic acids specifically derived from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PPD) and pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (DADE) and an imidization agent in a polar organic solvent, forming a film and then drawing and heat treating the film at a high temperature. The resulting polyimide blend film, however, has a rough, bark-like surface due to partial phase separation of the polyimide blend components during drying and curing.
Thus, a need exists for a polyimide blend film which has low water absorption, low coefficients of thermal and hygroscopic expansion and high mechanical strength and does not contain undesirable blisters or surface roughness due to phase separation of the polyimide blend components.